Projected tolerance zone mandatory requirement

[Andera]

Fig 6-11 from 2009 states: the feature axis must be within the specified tolerance of LOCATION over the projected height.

Should I conclude that if a position is used for the threaded hole it MUST have a projected tolerance zone?
Is it legal to use the position callout without projected tolerance zone (when a refinament of such used perpendicularity has one included)?

 

[Belanger]

When projected tolerance is used, it is most often modifying a position tolerance, so yes it's legal -- see Fig. 7-21 of that same standard. But you've found an interesting mistake in that figure's caption: Nothing shown on there is controlling location, so in the 2018 edition it was corrected to say orientation.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[Andera]

Belanger,

One additional question I have: for a thru threaded hole is it mandatory to show the projected tolerance zone with a chain line? Or is acceptable per y14.5 to use P modifier with the specified length in the FCF?

The main issue is how do you know which side of the thru threaded (again, not blind hole) the projected tolerance zone is applicable to if the "no chain line" approach is used?

 

[Belanger]

The projected zone projects away from the datum plane -- that's the key.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[Andera]

How do you know that? Where is specified?
How do you know is not thru the threaded hole? It is still "away" from the datum plane.

Also there are not example to show such of case, hence my question.

 

[chez311]

The projected zone projects away from the datum plane -- that's the key.

JP,

I didn't see any such verbiage in the standard. As OP noted "away" is imprecise and somewhat ambiguous.

In the case of a blind hole, or a through hole which is only threaded on one side we can rationally deduce from which side the projected tolerance extends.

There is actually an example in Y14.5-2009 of a fully threaded through hole fig. 7-22 where a heavy chain line is utilized to show the height and direction of the extension. I would consider this good practice in these cases when the direction may not be clear.

Also from the accompanying section 7.4.1.2 - "The direction and height of the projected tolerance zone are indicated as illustrated. The minimum extent and direction of the projected tolerance zone are shown in a drawing view as a dimensioned value with a heavy chain line drawn closely adjacent to an extension of the center line of the hole."

 

[ewh]

if a position is used for the threaded hole it MUST have a projected tolerance zone

True statement?

"Know the rules well, so you can break them effectively."
-Dalai Lama XIV

 

[chez311]

ewh,

I glossed over that part of the OP. As far as I can tell from the standard and my own experience I see no reason why that should be the case.

From Y14.5-2009 7.4.1 Projected Tolerance Zone: "The application of this concept is recommended where the variation in perpendicularity of threaded or press-fit holes could cause fasteners, such as screws, studs, or pins, to interfere with mating parts."

Not a mandatory requirement, just recommended. Not really sure where that reasoning came from, honestly.

 

[greenimi]

Not really sure where that reasoning came from, honestly.

My guess, .....because of the standard's mistakenly use of the word location instead of orientation.
What do you think?

But you've found an interesting mistake in that figure's caption: Nothing shown on there is controlling location, so in the 2018 edition it was corrected to say orientation.

 

[ewh]

Thanks chez, that was my opinion also... the more I do this work, the less it seems I actually know.

"Know the rules well, so you can break them effectively."
-Dalai Lama XIV

 

[Belanger]

I agree with the others -- it is not mandatory for threaded holes.

For my other comment (that it goes away from the datum plane), it's true that the standard doesn't say that explicitly. So it's OK to disagree with me

But I would ask why a projected tolerance would ever be projected toward the interior of the hole? If we understand the functional reason for a projected tolerance, then it kind of goes against the function to projected inward.

If you asking whether the projection can go outward, but from the face opposite the referenced datum, then we can discuss that too.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[chez311]

JP,

My point was mainly that the standard does not state explicitly what the "default" was, not so much that other interpretations would make much sense in most cases.

I would agree that for example in the case of Y14.5-2009 fig 7-22 even without the heavy chain line, the obvious conclusion is that the projected tolerance zone starts at the face referenced as datum feature A and extends outward from the material. I would just consider that in these cases (through hole - either unthreaded for a pin or fully threaded) its probably best to be as explicit as possible - if the OP is asking the question, I imagine others may as well.

Additionally, a handful of situations which are hopefully not too contrived/improbable come to mind where specificity as to the surface from which the projected tolerance originates and its direction would be beneficial - for example: a plate which has the width specified as a datum feature (FOS) instead of one side or the other and/or perhaps a through hole which has a pin or stud fully pressed/threaded through the part's width so that some significant amount protrudes through the other side.

 

[chez311]

greenimi,

Point taken. I initially thought that it was a bit of a leap in logic, but it also seems that just about all the examples of threaded holes in the standard are accompanied by a projected tolerance (unless I'm missing one) - which may reinforce such a perception, even though unthreaded holes can also have a projected tolerance applied and there is certainly no such requirement there.